A controversial chemical found in hard plastics seems to alter thyroid hormones that are essential for healthy growth, and the change is evident in pregnant women and newborn boys, a new UC Berkeley study reveals.

The substance, Bisphenol A or BPA, is widely used in plastic bottles, the linings of canned food and beverages, dental sealants and thermal paper commonly used for sales receipts. BPA has already been linked to increased risk for cardiovascular disease, miscarriages, breast and prostate cancer and reproductive dysfunction.

This latest study adds to the growing health concerns about the chemical, say UC Berkeley researchers, whose findings appear Thursday in the journal Environmental Health Perspectives. The team found changes in some thyroid hormones, which guide pre- and postnatal growth and brain development in humans, but says more research is needed to determine how those changes affect infants as well as their mothers.

"A couple studies have looked at BPA possibly influencing thyroid hormones, but nobody's looked at it in pregnant women," said Tracey Woodruff, director of UCSF's Program on Reproductive Health and the Environment, who was not involved with the research. "That makes the study important."

The study involved 476 pregnant women in Salinas who have been part of a long-running study of environmental health issues. Most were Mexican American immigrants who are under 30 years old, had low incomes and had given birth to more than one child.

Taking samples

The researchers analyzed BPA levels in women's urine samples during the first and second half of their pregnancies as well as thyroid hormone levels in blood samples from the pregnant mothers and from the newborns within a few days of birth.

Pregnant women with relatively higher levels of BPA in their urine had less of a thyroid hormone called total thyroxine in their blood, the team found. It is one of several thyroid hormones.

"Because thyroid hormones in the mom and the children are so crucial to normal growth and development," he said, "I would say any change in the thyroid hormones is substantially of concern and needs to be looked at really closely."

But Jeffrey Faig, clinical professor of obstetrics and gynecology at Stanford University School of Medicine, called the findings "not that profound."

Since most of the total thyroxine does not reach the fetus, its physiological effect cannot be determined, said Faig, who was unaffiliated with the study. A smaller part of the hormone that is known to affect the fetus' growth was not linked to the mother's BPA levels, he noted.

In addition, the UC Berkeley researchers found that newborn boys whose mothers had higher levels of BPA when pregnant showed signs of an overactive thyroid. Newborn girls did not show the same signs, however. One possible explanation for the gender difference may lie in studies that show female rats clear BPA more efficiently from their bodies than male rats, Chevrier said.

"In addition, since BPA is efficiently metabolized and rapidly cleared from the body," he said, "the limited BPA exposure measurements reported do not likely provide an accurate measure of maternal exposure to BPA during pregnancy." BPA's half-life in the body is about six hours.

BPA, which hardens plastics,is ubiquitous in the United States. About 2.4 billion pounds were produced nationwide in 2007, and about 90 percent of Americans have traces of BPA in their urine. In July, the U.S. Food and Drug Administration banned BPA from baby bottles and sippy cups, which the industry had voluntarily done for years in response to growing health concerns.

Chevrier said there is enough evidence against BPA that people should try to avoid canned food, baby bottles manufactured before the FDA's ban and liquid baby formula, all of which have been found to contain BPA.